Control system



.April 18, 1933. o. w.4 LIVINGSTON 1,904,485

. coNTRoL vSYSTEM'l l Filed sept. 11. 1931 Orfirj W. Livingas/ton,

` His Attorney.

"Patented pr. i8, v1933 l' UNITED STATES MNST owing; A

ORRIN W. LIVINGSTON, F SCHENECTADY, NEW YOR-K ASSIGNOB '130 GENERAL.ELECTRIC COMPANY,`A COB-BORATION' OF NEW YORK CNTRL SYSTEM Applicationled 4September 11, 393i; aerial No. 562,359.

This invention relates to control systems, more particularly toregulating systems, and it has for an object the' provision of a simple,reliable and improved system of the 5 kind. Morespecically the inventionrelates to regulating systems in which the regulated characteristic of acircuit is controlled by `means of electric valve apparatus, and accordingly a further object of this invention is the improvement andsimplification, of the circuit arrangement and apparatus em-. ployed tocontrol the valves.

Still morespecifically the invention relates to voltage regulatingsystems in which the voltage of 'a circuitis subject ,to wide variationsroma desiredvvalue responsively to changes in some operatingcondition ofthe circuit, such as the load, and a -urther objedtI electricyalves 'areemployed to maintain the f voltage of a translatingcircuit'substantially Iconstant at a desired value'. f n

Although not limited in ,its application i the invention is particularlyadvantageous in connection with systems in which the voltage of atranslating circuit is ycontrolled by means of a variable seriesconnected current limiting devicesuch for example as a theatre dimmingsystem in which the' intensity oilum'p illumination lof ,the variouslamp 'circuits is controlled by series connectedimpedances,suclpas'variable resistances or reactances.v In such a systemit is desirable especially in connection with the presetting of scenes,thatthe intensity of illumination of a lamp crcuit'vvill accurately`correspond with the indicationof a control device employed to vary thevillumination. The voltage drop across the series impedance device`changes, however, with changes in the lalnp' load, and consequently theposition ofthe indicator is not always 'an accurate "indication of thedegree of illumination. f

As an example, let it be assumed that the usual'or normal number oflamps are conf' nected in circuit and 'that the control device isactuated to a position indicating fifty liancy. If the number of thisinvention is the provision of an improved voltage regulating system inwhich.

of lamps in circuit is nbw decreased onehalf, the voltage drop acrossthe series impedance device will decrease proportionate` ly and/consequently the lamp voltage andl brilliancy Will increase to `a valuethat is considerably greater than the'lifty per centindicated by theindicating device. Similar-V ly if the lamp load is doubled, the voltagedrop across 'the series impedance will be correspondingly increased withthev result 4that the lamp voltage and brilliancy will decrease toavalue less Athan that indicated.

As previously pointed outfthis undesirable tor of the system can not,with any reasonable degree of accuracy, predetermine the brilliancyof acircuit for Widely differing Vlamp loads, "and this necessitates finaladjustment of the intensity of illumination after the scene has beenswitched on.

In prior systems this difficulty was partially overcome by utilizingspecially devsigned saturable reactors which due to the special designand construction vverel very expensive.

Accordingly an object of lthis invention isthe provision of a controlsystem in which means are provided for adjustingand maintaining the lampvoltage substantially constant at any desired value Withina Wide -rangeof lamp load. An advantage of the invention isthat a very `inexpensivetype of reactor may'be utilized instead of the ex-y pensive speciallydesigned reactors heretofore employed and since the reactor is the mostexpensive item in the lcircuit and a separate reactor is required foreach circuit,

thecost of` the entire system is .greatly re- ,.lued. A further``advantage of the invenvtion isl'that although the cost of the system isgreatly reduced the operating characteristics of the system are-greatlyimproved.

In'carrying the invention into eect in one form thereof, a' controldevice is employed to control an operating condition of an electriccircuit, and this device is conrtrolled responsively to the differenceof a control voltage and a voltage derived from the controlled circuit.More specifically, an

electric valve provided with a control electrode is employed to vary thereactance ot' Aa saturable reactor connected in circuit relationshipwith the controlled circuit vand this valve is in turn controlled bysupplying to the control electrode a control voltage, andija regulatingvoltage derived from the controlled circuit and of opposite polarity Itothat of the control voltage. This control voltage is variable to provideforadjusting the regulated characteristics to a desired value and theregulating voltaffe is preferably pulsating direct current rfor thepur.- pose of facilitating'its utilization to vary the bias of thecontrol electrode.

.In illustrating this invention in one form,

. thereof it is shown as embodied in a theatre and 'tothe accompanyingdrawing in which Fig. 1- is a simple diagrammati-cal representation ofan embodiment -of lthe Ainventign and Eig. 2 is a chart of curvesemployed o explain and clarify theI operation of the sys-` Referring nowto the drawinga suitable I translating circuit including a translatingdevice such as represented by the conductors 10, 11 and a plurality oflamps 12 is supplied from any suitable source such for eX- ample as thealternating current source represented by the conductors 13, 14. Thevoltage supplied tothe lamp and the degree of illumination thereof arecontrolled by means of. an impedance device, illustrated in thedrawingas. a saturable core reactorl, the reac ance 'of which iscontrolled. by suitable electric valve apparatus 16, the operau tion ofwhich in turn is controlled by a suitable. source of control voltage 17and a volt- `age-derived Afromithe load circuit 10, 11.

For the sake of simplicity the saturable reactor 15 is lshownconventionally in the drawing with the core member omitted.

This reactor comprises a plurality of reactance windings 18,19 each\wound upon a separate leg of the iron core and connected lin seriesrelationship between the supply source and the load. In the speciiicarrangement illustrated in thel drawing the reactance windings 18, 19are connected in parallel relationship with each other and in Leo-1,485

series relationship between the side 13 of,v 12. The saturable thesource and the lam reactor 15 is also provided with a control winding20. Direct current of a variable 'magnitude is supplied to the controlwindthe source 13, 14 remains substantially constant, a variation in thereactive drop across the .windings 18 and 19 effects a 'correspond-` ingchange in the voltage applied to the lamps 12 and consequently in theintensity of their illumination. It will thus be seen Y that a variationin the current flowing in the control winding 20 effects a correspondingchange in the voltage'applied to the lamp 12; an lincrease in thecurrent resulting .in a decrease in the reactive voltage .drop acrossthe reactance windings 18, 19

and an increase in the voltage applied to the lamp 12 whilst a decrease1n the current `results in an increase of the reactive drop across thewindings 18, 19 and a decrease of the lamp voltage; 'w

n Although the electric valvel apparatus 16 may be of any suitable typeit is illustrated in the drawing as an electric discharge device',preferably o f the three electrode type into the envelope of which asmall quantity of an Pinert gas such for example as mercuryvapor isintroduced after exhaust; the presence of the gas within the tubeserving lto change the usual pure electron discharge into an are streamconstituting the tube a grid controlled arc rectifier. The electricdischarge device 16 may be. either a full wave or a half wave rectifyindevice; the half wave rectifyin device ing` shown in the drawing for .te purpose of illustration due to the simplicity of the necessarycontrolling circuits. As shown, the electric rdischarge device 16 isprovided with an output electrode or anode 22, a control lelectrode -orgrid 23 and a ilamentary cathode 24;

the latter being heated to the necessary degree of incandescence byenergy supplied to it from' the secondary Awindlng 25 of a supplytransformer the primary winding 26 of which is supplied with energy4from any suitable source such as the alternating' current source 13, 14to whieh'iit is connected as illustrated. The anode 22 is connected toone side of the supply source over a circuit that may be traced throughthe conductor 21, the control winding 20 and conductor 26 to :the side14 of the suppl source.

Briefly stated Athe operation of the electric discharge device is asfollows; o

An alternating lvcltage is applied to the anode 22 fromvthe source 13,14 and thisl voltage is represented in Fig. 2 by the curve 27. Currentwillfflow in the output cir'- cuit, if at all, only during that halfcycle of anode voltage in which the potential of the anode is positivewith respect to that of the cathode 24, that is, only during the halflmust be at the proper value,`and this critical grid voltage, i, e., thevoltage at which current will just start flowing in the anode circuit isrepresented in Fig. 2 by the dotted curve 28.

in the output circuit it rises abruptly lo a value that is limited onlyby the impedance of the output circuit and continues until the outputcircuit is interrupted or until the anode voltage is reduced to zerowhich in the case o an alternating current ha e pens when the curve 27passes through t e zero point. The grid has no control of theinstantaneous value of the current in the outi Vput circuit but only hascontrol over the time in the cycle at which current starts t0 iow and itwill thus be seen that if the volt-' age applied to the grid is eitheran alternating or an undulating voltage that the average value of thecurrent flowing inthe output circuit may be controlledby shifting thephase of the'grid voltage relative to`that of the anode. When the gridvoltage is exactly in Lphase with the anode voltage the current willbegin to flow in the output circuit at the beginning of the positivehalt cycle and will continue until the anode voltage againpasses'through zero whilst if the grid voltage is 180 out of phase withthe anode voltage, current will notstart to' flow ai` in the outputcircuit at any timef) For intery.mediate phase relationships, currentwill start flowing at proportionate intermediate points in the halfcycle.

The grid'bias voltage may be derived from any suitable source 17 rwhichis illus-A trated in the drawing as al variable potentiometer resistanceone'terminal of'which is Yconnected to the side 13 of the suppl sourceby means of the conductor 29 and t e other terminal of which isconnectedto the opposite side ofthe supply source by means .of the conductor-30..The potentiometer 17 f is variableso that the control voltage su-pliedto the gridmay varied as `desired and. a. rectifyin'g device 31 ofvany suitable typeis included' in theconnections between` themovahlecontact 32o`fthe potentiometer 1ra-glatte teint se inthe grid ecircuitfor, the-purpose of rectifying fand applying av directfrurrentcontrolling voltage `the v.Wave the voltage applied to the Once currenthas started to flow grid., lt will be seen, l1cwever, that the rectifyindevice 35i onlyvrectiies onelial 'cycle of 'te voltage across thepotenticineter 17. In order that this halffwave rectilied voltage may beconverted into a voltage of N 'line 37 parallel to the axis 0 0.

A regulating voltage is derived 'treni the u voltage ef the lamp'circuit 10, 11 by ineens of lthe ltranstorifner 38-and is supplied thegrid 23 to which one terminal` of the sec ondary winding ci thetransformer 33 is connected as illustrated. The opposite terminal of thesecondary winding of the transformer 38 is connected to a secondelectrcde l of the rectiying device 3l and the polarity' of thetransformer winding is so chosen that the rectifying device 31rectifiesthe inverse half-cycley oi the lamp voltage wave, i. e.,

the half-cycle that isv opposite from that device 16: A suitable filterdevice compr1sing a reslstance 39`and a condenser/40 con-l nected inparallel therewith is' lconnected in the gridlcircuit one terminal ofthe resistance 39 being connected to the terminal .33 of the resistance34 and the ,opposite terminal oi" the resistance 39 being connected toone terminal of the secondary-winding Y' of the transformer 38. Theelectrical constants of the resistance 39 and the condenser 40 are sochosen that the rectified voltage is.,

which is rectified by'theelectric discharge slightly pulsating andtheconnections be- -tween the transformer 38, the rectifier 3l and theresistance 39 are so made that the u terminal of the resistance 39nearest the grid 23 is negative with respect to the point 33.

This Vregulating voltage"l which is supplied:

y the tothe grid/723 is represented in Fig. 2 pulsating curvea'l. i

lt will thus be ,see that the controlling and regulating voltages' arejoposite in polarity with respect to? the gri 23yand that, i

therefre, the actual or resultant grid voltage is the difference betweenthese two voltages and is represented in Wig. 2 by the dotted curve 41.f

The potentiometerjl? is` variable so thatv the magnitude of the controlvoltage can be varied at the will ofthe o erator and it 'will `'be clearby reference to magnitude of either the control volta e 37 ig. 2 thatifthe or the 're ulating voltage 40 is change this" i "will resu t' in'either raising or lowering the v thus effect a chan 'e in the i-currentowing in the output ciicult o f the electric discharge ydevice 16. I

Since the control voltage 37^is preferably a smooth direct currentvoltage it will be clear that this might be derived from a sep aratesource of direct current voltage instead of from the alternating currentsource 13, 14. The arrangementy shown in the drawing, however, ispreferable in that it eliminates the expense of a separate source.

Since the saturable reactor is highly inductive and since the electricdischarge device 16 is shown as a half-waverectifying .device it willbe. clear that very little current will flow in the output circuitthrough -the control winding20 because during the l 'inverse half cyclewhich the velectric discharge device is not rectifying the inductance ofthe reactor 15A would force current back into the line 13, 14 unlesssome other 'suitable vpath were provided. If current I ing the currentHow in the control winding were returned to the line 13, 14 during theinverse half cycle, the result would be that practically no directcurrent would ow'in the control winding 20 and the reactive'drop acrossthe saturabe reactor 15 could not be varied or controlled.

' For the purpose, therefore, of maintainduring the inverse half cycle,a suitable half- Lwavev rectifying devi'c'e 42 is connected in4 parallelwith the control winding; the anode v 43. being connected to the anode22 ofv the electric discharge device 16 andthe filamen- Vtary cathode 44being connected to the distant terminal of the control winding 20 andheated to the necessary degree of incandescence by currenty supplied toit from the secondary winding 45 of the supply transformer. It will beseen that when the voltage applied vto the anode 22 of electricdischarge device l6from the line becomes 'negative that the terminal ofthecontrol winding which is connected to the anode tends to remainpositive due to the inductance of the reactor 15 and consequently will I.continue to flow from the anode 43 to the inglthe current flow in thecontrol winding during the i verse half cycle this f current issubstantially a direct currentv of `constant.

but adjustable magnitude.

' With the above understanding of the apparatus and connectionscomprising an embodiment of the invention the operation will be readilygrasped' and easil7 understood from the description which f ws:

As a result of sustain-v charge device 16 `causes very .little currentto flow in the control winding 20 with the result that the reactice dropacross the vwindings 18, 19 is large and the lamp voltage is very small;so small in fact that the lamps are dark.

By moving the movable contact 32 to an intermediate position on thepotentiometer 17 the control voltage applied to the grid 23 is increasedto some positive value, such for example as that represented by thestraight line 37 in Fig. 2. The phase relationship of the actual gridvoltage relative to the anode voltage is thereupon suddenly changed toaposition in whichthe grid voltage is substantially in phase with theanode voltage 27 and as a result electric discharge device 16 allows alarge current to flow in the control winding 2Q thus reducing thereactive drop across the saturable reactor 15 and Vincreasing the lampvoltage. However, as the lamp voltageV is increased in magnitude theregulating voltage represented by the curve 4U is likewiselincreasedwhich resultsin shifting the phase of the actual grid voltage relativeto the anode voltage toward the ri ht to a position in which a balanceis establlshed between the positive control voltage and the nefrativ'eregulating voltage.

If the movable contact 32 is moved to the extreme .left-hand position,the magnitude of the control voltage 3.7 is 'further' increased in apositive `direction which results inshifting the phase of the actualgridvoltage further to the left to a position in phase with the anodevoltage as a resultof which the current in the control winding becomesmaximum, the reactive drop across the saturable reactor 15 becomesminimum, and the voltage applied to the lamps 12 becomes maximum and thelamps 4burn at maximum brilliancy.

In order to obtain-the regulating action it is of course necessaryfor'some condition such, for example, as the load of the-.lamp circuitto change. As an example let it be assumed that the movable contact ofthe potentiometer has been moved by the op.V

are connected in the circuit 10, 11 the lamps .will be illuminated atapproximately fifty n .per cent brilliancy. .Let it also be assumedthat-the operator desires only to have onehalf the usual or normalnumber of lamps in circuit and that the 1am load is reduced by one-half.This reduction in the lamp neonates load edects a corre/spendingreduction in translating device such, for example, as an the voltagedrop across the reactive Wind61 ings it? it and since the voltage 'o the.source i3, lli remains substantially the lamp voltage is increased laycor responding amount to a value that is dit# ferent from that indicatedon the potenti ometer. .iloWeaver9 as the lamp volta e in j creases, themagnitude oil the regu ating 1- vdltage represented lay the curves illand all.

increases in a negative direction the client of which, previouslyexplained9 is to shift tile phase oli tire actual voltage toward theright thereby effecting a den crease in the currenttlovvmg controlwinding 299 an increase in the reactive drop i across the saturahlereactor l5 and a decrease in the lampvoltaga' The phase oi" the gridvoltage will' continue to sliiiit tos 2- Ward .the right until thebalanced relation| ship between the regulating voltage and the controlvoltage is reestablishedI which conditioncan only obtain when the lampvoltage is at a propervalue, i.. e., the value L indicated hy theposition ci the movahle contact 32 on its associated indicating, de-

.viceu v Similarlyii the lamp load is doubled,

the reactive drop across the saturahle reac- .tor is increased and thelamp voltage de;l creased to a value that is lower. than the' valueindicated hy the position of the mov .able contact 32.011 its indicatindevice. decrease win the lamp voltage, owever," rei suits ina decreasein the magnitude of the regulating voltage with' a resultant shifttoward the left of the actual grid.v voltage7 .thereby eecting anincrease in .the current flowing in tire control winding 20, a decreasein the reactive drop of the saturahle reactor I15 and an increase in thelamp voltage@ is before the yactual igrid voltage continues to shiftuntila' balanced relationship between theregnlating voltage and thecontrol volt'- age :is reestablislied. at which time the lamp voltagewill loe such that the intensity oiil lumination is ati-the valueindicated by the Aposition of the movable contact 32 of theotentionieterv l?.

lt will thus be seen that oy this invention the lamp voltage andconsequently the intensity'oi illumination of the lamp is reguf Vlatedand' vmaintained suhstantiallyconstant at any desired value quiteindependently ci the lamp load and' that the operator can thus -beassured that the intensity of illumination of the lamp will accuratelycorrespond with -the position of the movable contact 32, as indicatedupon the associated indicatingde-A vice independently of the number oflamps in circuit spreviously pointed out, this invention is not limitedin its application to atheatredimmingsystem since it will be clear thatthe lamp load might be replaced by any electric motor and that thecontrol Winding 20 mig-lit loe any control device such, :for euample9 asthe ,delai 'Winding ci an electric motor or the ield Winding of a "WardLeoroA ard generator employed to supply any suitel able translatingdevice.

@although in accordance will the '"prmviu sions ci the patent statutes lleave described this invention as embodied in concrete torni, it shouldloe understood that the invention is not'limited to the preciseapparatus or exact connections shovvn in the. drawing since these aremerely illustrative and mod iiications and alterations will readily aug.gest themselves to persons skilled in the without .departing 'from thetrue spirit of this invention `or from the scope oi 'tl-ie au.-2 'nexedclaims.. D s

l claim as new and desire to' secure hyLetters Patent of the UnitedStates is: l. A control system comprising in comicin nation analternating current load circuit? control means for said circuitcomprising electric valve apparatus provided with a conn trol grid,means for supplying a varialol'e di rect current control voltage to saidgrid, and means comprisin a rectifying device ener-z gized from said aternatin'g current load cir' ling electric valve apparatus provided witha control grida means for supplying a variable direct current controlvoltage-tog said grid, and means comprising a rectiiying deviceenergized. irom said alternating current load circuit and connected incircuit with said grid 'so as to supply thereto a regulating voltagehaving a direct current com ponent ci opposite polarity with rewect tosaid control 'voltage u 3. A control system comprising an alternnatingcurrent load' circuit, `control means lfor said circuit comprisingelectric; valve apparatus provided 'mth a control grid-9 and `means forsupplying a pulsating direct current voltage to said grid comprising adevice connectedin circuit with said grid9 means Jfor supplying a directcurrent centrol voltage to said grid, and a 'jrectiiying deviceenergized from said alternating current load circuit and connected tosaid grid# sofas to supply thereto a regulating voltage having a dlrectcurrent component oi oppoliti site polarity vwith respect to saidcontrol voltage. 0 e 4. control system comprismg analternating currentload circuit, anvariable reactance device Ifor controlling the voltageof said eircuit, means for varying the reactance of said devicecomprising electric valve ap- P31' me s for supplying a variable phasepul- ,sating direct current voltage to said grid comprising a filterdevice connected in circuit with said grid, a rectifying deviceenergized from said alternating current -circuit and connected to saidgrid for 'supplyelectric valve controlled by said control vo'ltlage andsaid derived voltageA for controlling said reactance device.

In a control system for theatredimming apparatus and thelike, a lampcircuit, an impedance device for controlling the voltage of saidcircuit, asource of control voltage and means controlled by said lamp.

voltage and said control voltage for con` trolling. said impedancedevice.

7. A control system for theatre dimming apparatus and the likecomprising a lamp circuit, a variable impedance device included in saidcircuit for controllingthe voltage\ thereof, a source `of controlvoltage, means for deriving a voltage from said lamp circuit, and anelectric valve responsive to the difference of said control voltage andsaid derived voltage for varying the impedance of said impedance device.

-8. A control system comprising in combi nation, an alternating currenttranslating circuit,` means .for'controlliijig an operating' conditionof said circuit comprising an electric valve provided with a controlgrid,

means for supplying a bias to said grid, and

means responsive to said operating condition for supplyingl a slightlypulsating bias of opposite po arity to said grid. l

, n a control system, a load, means for supplying alternating voltage tosaidload; and means for maintaining the voltage of.

said load substantially constant at a desired value comprising anelectrievalve provided with a control grid, means for supplying a lvariable direct current bias to saidgridya'nd supplying a slightlypulsating bias of opposite polarity to said grid comprisinga filterdevice connected in circuit with.\ said grid and a rectifying deviceenergized from said'p. alternating load .voltage and vconnected to saidgrid. 10. A

control system comprising .in 'conitus comprising a control grid,andcuit for supplying to said gri a -bias of opi gosite polarity, andmeans for varying said rst mentioned. bias.`

11. In a control system, a source of alternating current, a load circuitsupplied from said source, means for controlling an operating conditionof said circuit comprising an electric valve energized from said sourceand provided with a contrbl grid, and phase shifting means forenergizing said grid comprising a source of bias voltage for said gridand/ means res onsive to variations in said operating con ition forsupplying to aid grid a pulsating bias of 'opposite poarit 12. In acontrol system, a. source of alternatingV current, a circuit including avariable load supplied from said source, means for controlling thevoltage of said load com prising an electric valve'energized from saidsource and provided with a control grid, and phase shifting means forenergizing said grid comprising a variable source of bias and meansresponsive to variations in said load for supplying to said grid apulsating bias of opposite polarity. A

, 13. A control system comprising a source of alternating current, a.translating circuit supplied from said source, means for varyingthe'voltage supplied to said circuit comprising an electric valveenergized 'from said source and provided with a control grid, and phaseshifting means for energizing said rid comprising a source of directcurrent bias for said grid, and a rectifyng device energized from saidcircuit and g filter device connected therewith for applying to'saidgrid a pulsating bias of opppste polari t 14. Illa regulating system, aLsource of alternating current, a load supplied from said source, voltagecontrol means forsaid load comprising a saturable reactor provided with'a control' winding, means for controlling the reactance `o -isaidcomprising anelectric valve connected' to said.. control winding and'/energized from laid source, said valve being provided with a-controlgrid, a variable lpotentiometer conmeans responsive to said vloadvoltage 'for' nectedito said source, and means for applying a pulsatingbias to said grid comprising a rectifying -device connected to saidpotentiometer and to said load, and a filter device connected to saidgrid.l

saidrectifying device and to control'system, a source of alternatingcurrent, a load supplied from said source, voltage control means forsaid load comprising a variable saturable reactor connected in seriesrelationship with said load and provided with a control Winding, means:tor varying the reactance of said reactor comprising a half-waverectifying electric valve energizedfrom said source and connected tosaid control Winding, said -valve being provided with a control grid, asecond valve connected to said control winding for sustaining thecurrent flow during the inverse half-cycle, and phase shifting means forsaid grid comprising a rectifying device connected to said load and tosaid source for supplying to said grid a bias derived from the inversehalf-cycle of said load voltage and a bias of opposite polarity, filtermeans for rendering said deing bias of opposite polarity and having avariable magnitude proportional to said lamp circuit voltage, and avariable potentiometer for varying said first mentioned bias.

19. A Icontrol system for theatre dimming apparatus comprising a sourceof alternating current, a lamp circuit supplied from said source, meansfor vai'yiiig the voltage of said circuit comprising a saturable reactorprovided with a control Winding, means for varying the reactance of saidreactor comprising an electric valve energized from said source andconnected to said control Wiiiding, said valve being provided With acon-trol grid, and phase shifting means for energizing said gridcomprising a source of grid bias voltage, a variable potentiometer rivcdbias slightly pulsating and said oppofor varying said bias voltage, andmeans site bias substantially non-pulsating, and a comprising ahalf-wave rectifyiiig device en.- variable potentiometer for varyingsaid opergized from said lamp circuit and a filter posite bias.

16. In a control system for theatre dimming apparatus and the like, asource ot supply,va lamp circuit supplied from said source; and meansfor varying Vand maintaining the voltage supplied to' said circuitsubstantially constant at adesired value, comprising an impedance devicefor controlling thel voltage of said circuit, an electric valve providedWith a control electrode for varying the impedance of said device, meansfor supplying a bias voltage to said electrode, means'responsiveto thevoltage of said lamp circuit foi-.supplying a bias voltage of oppositepolarity to said electrode, and means for varying said first men' tionedbias.

17. A control system for theatre dimming apparatus comprising analternating current source, a lamp circuit supplied from said'source,avariable reactance device for varying the voltage of said lamp circuit,means for varying the reactance of said device comprising an electricvalve energized from said source and provided with a control grid, phaseshifting lmeans for energizing said grid comprising a source for sup?plying a bias to said grid, and -imeans responsive to variations intheload of said circuit for supplying a variable bias of oppositepolarity to' said grid.

18. In a .control system for theatre dimming apparatus and the like, asource of alternating current, a lamp circuit supplied' from saidsource, a variable saturable reactor included in said vcircuitl forvarying the voltage thereof, means for varying the reactance of saidreactor comprising an electric valve energized from said source andprovided With a control grid, phase shifting 'means for energizing saidgrid comprising a source for supplying a bias to said grid, meansresponsive to the voltage of said circuit for supplying to said grid apulsatdevice connected to said rectifying device :tor applying .to saidgrid a pulsating bias lvoltage of opposite polarity to said first myhand.

ORRIN W. LIVINGSLON.

